Just as the name suggests, a Russian-language domain name is such a domain name that is decoded as a combination of characters of Russian alphabet (as well as digits and hyphens) according to RFC-3490, 3491, 3492, 3454. Russian-language domain names start with a character string "xn--" followed by Latin characters, digits and hyphens.
In other words, a Russian-language domain name is a domain name that starts with a character string "xn--", whereas a corresponding word consisting of Russian characters is just its representation.